EP0225512B1

EP0225512B1 - Digital free-running clock synchronizer

Info

Publication number: EP0225512B1
Application number: EP86115966A
Authority: EP
Inventors: John G. Theus
Original assignee: Tektronix Inc
Current assignee: Tektronix Inc
Priority date: 1985-11-29
Filing date: 1986-11-17
Publication date: 1990-11-07
Also published as: DE3675506D1; CA1284363C; JPS62131631A; EP0225512A1; US4691121A

Description

Background of the Invention

The invention relates to an oscillator circuit as mentioned in the preamble of patent claim 1 which is already known from ELECTRONIC No. 12 (1978) pages 82/84:

"VerzögerungsIeitung - Anwendungen eines analo- gen Bauelementes in schneller Logik".

Computer systems are required to work efficiently with data stores of different speeds within the same system. If a semiconductor data store is used, the characteristics of the dynamic storage devices require that data in the storage elements be refreshed periodically. If store refresh operations are accomplished internally of the data store, the response time of the store will vary, and, accordingly, fully synchronous operation of a data store and the system central processor is not practical or desirable. Nevertheless, data transfer between the two units must be synchronized. One means of effecting synchronization between asynchronously operating units of a computer system involves resynchronization or restarting of a clock signal source of one unit, for example a free-running oscillator forming a part of such unit, with a clock signal from another unit. However, clock synchronizing circuits using logic gates are particularly susceptible to logic race conditions inasmuch as the asynchronous clock signals of the various units of the system drift with respect to each other. Such logic race conditions can result in the generation of clock signals having pulse widths of insufficient duration for proper system operation.
Accordingly, it is an object of the invention to provide an improved synchronizing circuit for a free-running clock signal generator.
Another object of the invention is to provide an improved free-running clock signal generator having a digital logic circuit for synchronizing the generation of the clock signals with an asynchronous clock signal from an external source.
Another object of the invention is to provide an improved digital-logic clock signal synchronizing circuit which provides protection against logic race conditions.

Summary of the Invention

In accordance with the present invention, the oscillator circuit as mentioned in the preamble of patent claim 1 is constructed as it is mentioned in the characterizing part of patent claim 1.
A preferred embodiment of the oscillator circuit according to the present invention is subject matter of the patent claim 2.

Brief Description of the Drawings

While the invention is set forth with particularity in the appended claims, other subjects, features, the organization and method of operation of the invention will become more apparent, and the invention will best be understood, by referring to the following detailed description in conjunction with the accompanying drawings in which:

FIG. 1 is a logic diagram of a prior art free-running clock signal generator of the type utilized in the present invention,
FIG. 2 is a logic diagram of a free-running clock signal generator in accordance with the instant invention, and
FIGS.3 and 4 are timing diagrams useful in explaining the operation of the present invention.

Description of the Preferred Embodiment

Referring now to the drawings for a more detailed description of the construction, operation and other features of the instant invention by characters of reference, Fig. 1 shows a free-running oscillator circuit comprising a NAND logic element or gate 10 having an input terminal 12 receiving an asynchronous control signal CS. An output terminal 14 of the NAND gate 10 is connected to an input terminal of a delay element 16 having a time delay of T2, while an output 18 of the delay element 16 is connected as a second input of the NAND gate 10. When the CS control signal is enabled or high, the oscillator runs but when the CS signal is disabled or low, the oscillator output signal KS at the output terminal 14 of the NAND gate 10 remains high. The CS control signal therefore turns the oscillator on and off or synchronizes the oscillator output with the rising edge of the CS signal. A first transition from high to low of the KS output signal follows the transition of the CS control signal from low to high by one gate delay, the delay of the NAND logic element 10. Subsequent transitions of the KS clock signal occur after delay T2 plus the gate delay of the. NAND gate 10. The oscillator circuit of FIG. 1 represents the prior art which has a disadvantage of possible unstable operation due to a logic race condition when the CS control signal changes state at the same time the KS output of the NAND gate 10 changes. At such time a KS clock signal having less than acceptable pulse width can occur.
Referring now to FIG. 2 in conjunction with the timing diagrams of FIGS. 3 and 4, a free-running clock synchronizer circuit in accordance with the present invention includes a transparent latch circuit 30 receiving a CS1 control signal on an input terminal 32. The latch circuit 30 may comprise a conventional storage element such as a D bistable or flip-flop. An output terminal 34 of the latch circuit 30, signal CSL, is connected as one input of a NAND logic element or gate 36, the output of which is the clock signal KS1. Clock signal KS1 is coupled to input terminal 37 of delay line 38 adapted to produce a first output at tap 40 after a time delay T1. This first output at tap 40 of the delay line 38, signal DL1, is connected as a first input of an AND gate 41 while a second output tap 44 of the delay line 38, signal DL2, is connected as a second input of the NAND gate 36. The delay at the second output tap 44 of the delay line 38 is T2, T2 being greater than T1. The CSL signal is connected as a second input of the AND gate 41, an output of which is supplied as a first input of a NAND gate 42. Tap 44 of the delay line 38 further provides a second input to WAND gate 42 via an inverter 46, and NAND gate 42 supplies a signal DLE at terminal 48 which is connected to a latch-enable input 50 of the latch circuit 30.
The KS1 signal output of the WAND gate 36 is a square wave clock signal with a period 2 (T2 + D1) where D1 is the gate delay of the NAND gate 36. The latch circuit 30 functions normally as a transparent logic element that passes the CSI control signal directly. with only one gate delay, to the NAND gate 36 as control signal CSL on terminal 34. The CSL signal gates the KSI oscillator output signal on and off so the oscillator output signal is synchronized with the CS1 control signal.
The enabling (high) output at terminal 48 of the NAND gate 42 normally enables the latch circuit 30 to pass the CS1 control signal as the CSL signal. But after the positive portion of the KS1 signal has traversed the delay line 38 and appears at the T1 output tap 40 as the DL1 signal, the CSL and DL1 signals enable the AND gate 41 which causes the DLE signal output at terminal 48 of the NAND gate 42 to go low for a period T2-T1 and disable latch 30, i.e. when a positive-to-negative transition of the KS1 signal may occur. As shown in FIG. 3, the CS1 signal might also be in transition from high to low during such period. The disabled latch circuit 30 then protects the oscillator circuit from an undefined or unstable logic condition of NAND gate 36 caused by the CS1 control signal changing state at or near the same time the KS1 output of the NAND gate 36 changes. In particular, the latch circuit 30 prevents KS1 from returning immediately to a positive level until after the DLE signal concludes and latch 30 generates the falling, edge of CSL. The circuit output is then synchronized with the rising edge of the CSL signal so that delaying the generation of the falling edge of the CSL signal does not adversely affect circuit operation. It is seen, however, that the circuit is prevented from generating pulses shorter than a given duration.
FIG. 4 is an expansion of the potentially unstable period when the DLE signal is low, and shows the gate delays associated with the generation of a minimum-width negative transition of the KS1 clock signal. The protection circuit comprising the gates 41, 42, 46 and the latch circuit 30 ensures that a negative portion of the KS1 signal will have a length of at least three gate delays.

Claims

1. A free-running clock synchronizer circuit, comprising:

a coincidence logic element (36) having a first input receiving a control signal, a second input, and an output, the control signal having first and second logic states;

and a delay element (38) having an input terminal connected to the output of said coincidence logic element (36), and a first output terminal connected to the second input of said coincidence logic element (36), said delay element (38) passing a signal on the input terminal to the first output terminal after a delay T2, the control signal in the first logic state enabling said coincidence logic element (36) to regenerate on the output a complement of the signal on the second input, thereby generating on the output of said coincidence logic element (36) a square wave output signal, the control signal in the second logic state disabling said coincidence logic element (36) to stop the operation of said oscillator, said delay element (38) having a second output terminal passing the signal on the input terminal after a delay T1 where T1 < T2, characterized by a latch circuit (30) receiving an asynchronous timing signal from an external source and having an enabling input, said latch circuit (30), when enabled, regenerating the asynchronous timing signal as the control signal, and a logic element (42) having an output connected to the enabling input of said latch circuit (30), said logic element (42) receiving the first and second outputs of said delay element (38) and generating an enabling output signal, the enabling output signal being disabled during a period of positive to negative transition of the square wave output signal.

2. The free-running clock synchronizer circuit of claim 1, wherein said coincidence logic element (36) comprises a NAND gate and the square wave output signal has a period 2 (T2 + D) where D is a gate delay of the NAND gate.